The invention relates to thin film solar cells. More specifically, the invention relates to thin film amorphous silicon solar cells.
Photovoltaic cells are capable of converting solar radiation into usable electrical energy. The energy conversion occurs as a result of what is well known in the solar cell field as the photovoltaic effect. Solar radiation impinging on a solar cell is absorbed by the semiconductor layer, which generates holes and electrons. The electrons and holes are separated by a built in electric field, for example, a rectifying junction such as an NIP junction, in the solar cell. The electrons flow towards the N-type region and the holes flow towards the P-type region. The separation of the electrons and holes across the rectifying junction results in the generation of an electric current known as the photocurrent and an electric voltage known as the photovoltage.
Various types of solar cells are being actively researched to create devices which can effectively compete with conventionally generated forms of electricity. The research is focused on two main alternatives. One research area has focused on small area high efficiency single crystal silicon or III-V compound solar cells such as gallium arsenide. The III-V cells are used in conjunction with a concentrating lens. The other main research area has focused on inexpensive thin film solar devices such as Cu.sub.x S/CdS or hydrogenated amorphous silicon or hydrofluorinated amorphous silicon.
This invention focuses on thin film hydrogenated amorphous silicon solar cells. G. A. Swartz, J. Appl. Phys.,vol. 53(1), January 1982, pp 712-719, teaches that the major loss mechanism in an N.sup.+ IP.sup.+ amorphous silicon solar cell is the back diffusion of holes into the thin N.sup.+ -type region (the first letter, (N.sup.+), indicates the region through which solar radiation first penetrates the solar cell). More generally, this loss mechanism applies to any solar cell wherein the light first enters the cell through a region which ohmically contacts the top electrical contact. Thus, it would be highly desirable to have an amorphous silicon solar cell which incorporated a means for minimizing or eliminating the major loss mechanism and a method of reducing or eliminating the back diffusion of holes into the thin N.sup.+ -type region of the solar cell.